Linux Process Management: Using lsof to List Open Files
lsof Reveals All
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME sshd 2354 juliet mem REG 254,0 14880 105723 /lib/libcap.so.1.10 sshd 2354 juliet DEL REG 0,8 127123574 /dev/zero bash 2363 juliet cwd DIR 254,4 20480 7274497 /home/juliet bash 2363 juliet txt REG 254,0 769368 4126 /bin/bash bash 2363 juliet mem REG 254,0 97928 105698 /lib/ld-2.3.6.soThe FD column shows file descriptor information, or identifies other types of file. Here, cwd indicates the current working directory, and txt indicates program text. The TYPE column has filetype info (REG indicates a regular file). The NODE column may be useful if you're trying to recover a deleted file. See the man page for a full explanation of the output.
lsof filename shows which processes have files of this name open. lsof +D /directory will show processes which have files in this directory open. You can use this if you're trying to unmount a filesystem but getting an 'in use' error, to find the processes using files on that FS and kill them as required.
lsof -c processname will show all processes beginning with processname that have files open; lsof +p PID does the same thing for a process ID. Using lsof -i will get you information about IP sockets. Check out the man page for more detail and for the many other available options.
Article courtesy of ServerWatch